Studies of UV radition effects of organisms have been focused on pyrimidine dimers and have shown the efficient repair of these dimers in normal cells. Since damage once repaired presumably has no untoward effects on cellular functions, it must be either nonrepairable or inefficiently repaired damage that is responsible for the apparent cellular UV sensitivity. Therefore, evaluation of the production, repair, and lethality of UV damage other than dimers, i.e. non-dimeric damage becomes vitally important. The aim of this research to accumulate further knowledge pertinent to the chemical characteristics of pyrimidine photohydrates, pyrimidine adducts, pyrimidine coupled products, pyrimidine dimers, and pyrimidine epoxides as well as the newly discovered photoproducts isolable from ultraviolet irradiated DNA. Such information is essential for the complete understanding of ultraviolet irradiation effects of cellular DNA.